## Fundamental Technologies## Galileo Spacecraft Pages |

*Source: JOMPI Proposal, November 1976*

The CMS is a three parameter (time-of-flight, dE/dx and E) solid state detector telescope
designed to measure with high resolution the elemental composition, energy spectra and pitch angle
distribution of the Jovian magnetospheric Z>=2 energetic particle environment throughout
the Orbiter mission. The telescope consists of a mosaic of three very thin front elements
(2 and 5 micron thick totally depleted surface barrier detectors) that are separated by a 7.5 cm
time of flight path from the rear detectors. Incident particle energy loss (dE/dx) is measured in
the front element, particle time-of-flight (TOF) is measured between the front and rear detectors, and
residual energy is measured in the rear detector. Three parameter analysis is thus
performed on particles penetrating the front detector, providing accurate elemental resolution
with high immunity to accidental events over an energy range (for Na ions, for example) from H100 keV/nuc
to >10 MeV/nuc. Single parameter rate channels extend coverage for
medium-Z nuclei to 20 to 50 keV/nuc, and unique two and three parameter
particle identifier circuits, based on logarithmic amplifiers, provide rate
data and priority information with excellent temporal and angular
resolution. An adaptive electronic priority system (utilizing the
particle group identifier channels) establishes priority for full three-parameter
analysis on the basis of atomic number, thus assuring adequate sampling
for all species, over the full range of nuclear charge and energy.
These proven design factors, plus a low energy threshold, a good geometry
factor (total G = 0.01 cm^{2} ster. for three-parameter analysis and = 0.31
cm^{2} ster. for single parameter analysis), and wide dynamic range allow
the CMS to accurately analyze the composition of the Jovian
radiation environment from the low fluxes expected at apojove and off the
equatorial plane to the regions of intense flux at the orbits of
Io and Ganymede.

*Source: Ted Fritz, SEL Memo, April 24, 1978 *

During the March 15-21 meetings held at MPAE I was given the action item of calculating the geometric factor of the CMS telescope design as schematically outlined by Herr Bolme. Upon returning from Germany I discussed the problem with Harold Leinbach, and he agreed to use his computer program to make these calculations. The resultant geometric factor for the front elements is:

Detector | Area (mm^{2}) |
g (cm^{2} ster.) | Max
q/Z* |

Ja | 25 | 0.1044 | 28 degrees |

Jb | 25 | 0.1044 | 28 degrees |

Jc | 5 | 0.0211 | 24.5 degrees |

Total J (single parameter) | 0.2299 |

Geometric factor of detector K (100 mm^{2}).

Through detector | g (cm^{2} ster.) |
Max q/Z** |

Ja | 0.00497 | 6.8 degrees |

Jb | 0.00497 | 6.8 degrees |

Jc | 0.00099 | 5.5 degrees |

Total K (coincidence) | 0.01093 |

*This angle represents the largest possible half-angle at which a particle still strikes the J detector, relative to the axis of the collimator.

**This angle represents the largest possible angle at which a particle still strikes the K detector after passing through the respective J detector, relative to an axis through detectors Ja, Jb, or Jc.

These numbers are in reasonable agreement with the
proposal values of 0.31 cm^{2} ster. (single parameter) and 0.01 cm^{2}
ster. (coincidence).

__Note on Geometric Factor for 0 Degree End in CMS Unit__

*Source: B. Wilkin, MPAE, July 24, 1979*

The 5m/50 mm^{2} telescope in the DE/E-telescopes is
designed with a 45º opening angle centered on the surface of the back
detector. The total collimator length of 21 mm (from surface of front
detector) defines then a collimator angle of b = 18.5º. (Any growth of the
collimator length l will be done with b = 18.5 =
const.) The collimator ratio is presently l/r = 5.4. (According to T. Mueller it
can be increased to 6.0.)

The differential geometric factor dG/da is shown in Figure 1-13 as a function of the angle a (curve (2)). (a denotes the angle of incidence formed by a parallel beam with the collimator center line; compare Figure 1-14). In Figure 1-13 the effect of the collimator is described by curve (3), curve (1) is the dependence of dG/da with no collimator at all. The function dG/da reaches the maximum value (modal value) at an angle somewhat larger than the collimator angle b = 18.5º. a(mode) = 200 dG/da drops to zero for the maximum angle accepted by the collimator agr=dG/da drops to zero for the maximum angle accepted by the collimator agr = 35º. (Note: Figures 1-13 and 1-14 are not available.)

It turns out that the median value am for the distribution dG/da is pretty close to
b: am = 18.5º.
The median value of the detector thickness corresponding to
am is then: dm = 1.054 d. If we define a FWHM value for the distribution dG/da we
find FWHM - 20.5º corresponding to an effective range for dm:
dm = (1.054 +/- 0.05) d, i.e., the effective thickness of the detector is
increased by 5%. The maximum thickness occurs for a beam with
a=agr=35º: d_{max} = 1.225 d.

Figure 1-15 shows the effect of the thickness
variation in the DE/E curves of a 5m detector. The FWHM variation seems to be comparable
with the other statistical fluctuations in the
signal. This effect seems to be acceptable. The upper extreme limit, on the other hand, is quite
large and may in fact generate some minor overlap. However, d_{max} occurs
only with a very small probability. (Note: Figure 1-15 is not available.)

Conclusion: The large opening in the CMS DE/E telescopes seems to be an adequate compromise if a large geometric factor is the more important feature. Some additional spread is to be expected from the large range permitted for the angle a. The FWHM fluctuation introduced by thickness variations seems to be comparable with contributions from other noise sources in the DE/E system.

According to T. Mueller, the total length of the CMS unit can be increased to 150 mm. As a result we modified the backlooking (dual telescope) collimator as follows:

l = 300 mm (surface front det.-edge of collimator)

b = 16.74º

amax = 22.5º

agr = 30º

The angles are somewhat smaller than before and this will reduce the signal spread introduced by thickness variations.

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Updated 1/4/05, T. Hunt-Ward

tizby@ftecs.com